Transcript Astronomy news

Dying Star Reveal More Evidence for
New Kind of Black Hole NASA, 01.05.06
The orbital Period of the
Ultraluminous
X-ray Source in M82
Philp Kaaret, Melanie G. Simet,
Cornelia C .Lang; Jan. 27;
BREVIA, Science.
NASA's Chandra X-ray Observatory captured this image of the central region of the starburst
galaxy M82. From 1999-2000, this source was seen by Chandra to increase dramatically in
intensity over a period of three months (compare right and left panels), an indication of black
hole activity. With NASA's Rossi Explorer, a team led by Philip Kaaret has found evidence
of a star (too small to be imaged) orbiting around the suspected black hole, called M82 X-1.
This may confirm that M82 X-1 is part of a new "in-between" category called intermediate-mass
black holes.
Intermediate-mass black holes in M82 ?
Masses in the range 102 to 104 M⊙--would be a new class astrophysical object and may be
important in the formation of supermassive black holes.
The brightest X-ray source in the nearby starburst galaxy M82 has apparent luminosity
reaching 1041 erg s-1, indicating a mass exceeding 500M⊙. The source also shows quasiperiodic x-ray oscillations at frequencies of 0.05 to 0.1 Hz, suggestive a mass near 100 to
1000 M⊙.
Intermediate-mass black holes in M82 ?
They monitored the x-ray emission from M82 every other day for 240 days with the PCA
on the RXTE satellite. They found that the x-ray flux from M82 is modulated with a peakto-peak amplitude corresponding to an isotropic luminosity 2.4x1040 erg s-1 in M82 and a
period of 62.0 ± 2.5 days.
Assumption
X-ray modulation at the orbital period are known from several black hole x-ray binaries.
The high inferred luminosity indicates that the black hole is gravitationally pulling the mass
directly from the outer surface of its companion star. Because of the geometry of the binary
system, the orbital period is related to the companion star density ρ~115 Phr-2 g cm-3.
The mean density of the companion star is 5x10-5 g cm-3, which is compatible with giant or
supergiant star.
The ULX in M82 lies close to and possibly within the super star cluster MGG 11. An
intermediate-mass black hole may have been formed by stellar collisions in the extremely
dense core of MGG 11.
When the companion star of an intermediate-mass black hole evolves through the giant
phase, mass transfer causes the orbit to widen. The lifetime of this phase is short, of order
105 years, compared with the companion age 107 years.
Conclusion
"There has been debate about the masses and how intermediate black holes would form.
RXTE has provided major new insight." said Dr. Jean Swank.
"We caught this otherwise ordinary star in a unique stage in its evolution, toward the end
of its life when it has bloated into a red giant phase. As a result, gas from the star is spilling
into the black hole, causing the whole region to light up." said Prof. Philip Kaaret.
"We found a rise and fall in X-ray light every 62 days, likely caused by the orbit of the
companion star around the black hole. Because the star is located in such a dust-obscured
area, this makes it hard for optical and infrared telescopes to observe the star and velocity
calculations." Said Simet.
"With the discovery of the orbital period, we now have a consistent picture of the whole
evolution of a mid-mass black hole binary". said Kaaret. "It was formed in a 'super' star
cluster; the black hole then captured a companion star evolved to the giant stage; and we
now see it as an extremely luminous X-ray source because the companion star has expanded
and is feeding the black hole."